The _heapset function shows free memory locations or nodes that have been unintentionally overwritten.

_heapset checks for minimal consistency on the heap and then sets each byte of the heap's free entries to the fill value. This known value shows which memory locations of the heap contain free nodes and which contain data that were unintentionally written to freed memory. If the operating system does not support _heapset(for example, Windows 98), the function returns _HEAPOK and sets errno to ENOSYS.